Other DIY Testers
Simple circuit continuity tester
You can build a simple circuit continuity tester with just a few parts. This would allow testing a cut or broken extension cord to discover which wire connects to the wide terminal (common) and which wire is the narrow terminal (hot) at the plug end. (Of course, disconnect the extension cord from the wall socket first!). You could also use it to test the ends of a communication cable to discover which wire connects to which pin. If there's "continuity" (i.e. "a connection") then the LED would light. If not, the LED would remain dark. A continuity tester can also be used to check various electrical hardware components such as switches, physical connectors, plugs and jacks, etc. However, you can't use a continuity tester on everything. For example, don't use it to check a PIC chip, transistor, diode or other sensitive electronic part because they can be damaged by the tester's voltage.